Braking systems are known in which, in at least one operating mode (for example in an operating mode in which the wheel brakes are controlled electrically as a function of the driver's braking input), the pressure medium is enclosed (cf. for example German Patent No. DE 40 37 662). In the case of long braking operations, for example continuous braking downhill, the volume of the enclosed pressure medium expands as a result of heating in the wheel brake cylinders. Because the enclosed volume generates a minimum braking pressure, it is possible for the thermal expansion in volume to become so great that the pressure in the wheel brakes can no longer be reduced to very low pressures. It is an object of the present invention to ensure that brake pressure can be reduced to very low pressures even when braking continues for an extended period in braking systems having a trapped volume of brake fluid.
SAE Paper 960991, Electrohydraulic Brake System--The First Approach to Brake-By-Wire Technology, by Wolf-Dieter Jonner, Hermann Winner, Ludwig Dreilich and Eberhardt Schunck, describes an electrohydraulic braking system in which the driver's input is detected, for example by means of a pedal travel and/or by means of a pressure sensor; setpoints for the brake controller are formed therefrom; and the pressures at the wheel brakes are set so that the actual values approach the setpoints. In the event of failure of this electronic controller, hydraulic or pneumatic pathways are opened for a conventional direct action by the driver on the wheel brake pressure. Enclosed pressure medium volumes which can lead to the aforementioned disadvantages can also occur with a braking system of this kind.
It is also known from European Patent No. EP 489 887 to estimate the temperature of a wheel brake.